This invention relates to a process for the delignification of cellulose-containing; namely, lignocellulose-containing raw materials with a pulping solution of concentrated aqueous monocarboxylic acid, acetic acid, at increased temperature and under elevated pressure.
The classical processes utilized on a large industrial scale for obtaining pulp from the typical raw materials, namely suitable hardwoods or softwoods, annual or perennial fibrous plants, such as, for example, bagasse, reed or Miscanthus sinensis, as well as cereal straw, are the sulfite process and the sulfate process. The pulping liquors utilized in these methods exhibit well-known drawbacks with respect to the inherent pollution. These drawbacks are, on the one hand, the use of sulfur-containing pulping compounds and the problematic exploitation of the resultant sulfur-containing waste liquors which include the separated lignin and on the other hand, the use of chlorine-containing bleaching agents which are not harmless, either.
A course deviating from the sulfite process or the sulfate process was followed with processes utilizing organic solvents in place of the polluting inorganic chemicals as the pulping agents. Acetic acid, in particular, proved to be advantageous in this respect. The prior art includes pressure-less processes for obtaining cellulose pulp from lignocellulosic starting materials by means of acetic acid in the presence of catalytically acting amounts of mineral acids, as well as non-catalytic processes for the manufacture of cellulose from lignocellulosic materials by means of acetic acid, as described in U.S. Pat. No. 3,553,076 according to which temperatures are employed on the order of 150.degree. to 205.degree. C. Under optimal conditions, it is possible by means of these methods, for example with softwood as the starting material, to reach residual lignin contents of 3.4 to 6.8% by weight, corresponding to kappa numbers of 20 to 40.
A further drop in lignin content either was not desired or was obtained by means of using conventional chlorine-containing bleaching chemicals.
In another process for wood pulping with acetic acid, as disclosed in DE 3,445,132 A1, the pulping solution contains, besides concentrated acetic acid, also minor proportions of a mineral acid, e.g. hydrochloric acid, used as a catalyst. Furthermore, an extraction with dilute aqueous sodium hydroxide solution has been disclosed; this would require a technically expensive alkali recovery. Furthermore, the continuous percolation of the wood as provided, for example, in DE 3,445,132 A1 is accompanied by high acetic acid circulations, a weight ratio of wood to acid solvent of about 1 to 20 being typical.
In the process according to DE 3,445,132 A1, acetic acid is utilized in the boiling stage for the digestion of comminuted wood or of annuals as the starting material, as well as in a subsequent bleaching stage in the presence of hydrogen peroxide.
However, ozone has also been employed successfully as a bleaching agent in acetic acid, as described in EP-A-0,325,891.
On the one hand, the catalytically active amounts of mineral acids can hardly at all be recovered from the resultant wastewaters of, the dissolved wood decomposition products. On the other hand, these mineral acids are volatile, e.g. hydrogen chloride or hydrogen bromide, leading, inter alia, to odor problems. The salts formed from the inorganic components initially present in the pulping material or produced in case of an optional neutralization with dilute aqueous sodium hydroxide solutions would increase the burden on the wastewater and, on account of the halogen content, would restrict utilization and/or waste removal of the separated lignin. Furthermore, as is known, the presence of hydrogen halides in the aqueous phase is accompanied with considerable corrosion problems in connection with the container or processing materials.
These deficiencies have proved to be impediments for the large-scale industrial conversion of the process according to DE 3,445,132 A1 or EP-A-0,325,891.
Moreover, these processes are characterized by high consumptions of bleaching chemicals with comparatively low brightness values in the product. Thus, for softwoods, in the process according to DE 3,445,132 A1, consumptions of hydrogen peroxide can be expected of about 5% by weight based on pulp in absolutely dry state (hereinafter designated as "atro"), with a final brightness of 48.6% ISO or, in the process according to EP-A0,325,891, a consumption of about 2.6% by weight of ozone and of 1.0% by weight of hydrogen peroxide can be expected based on atro pulp, with a final brightness of 62% ISO.